Leopold II (French: Lopold Louis Philippe Marie Victor, Dutch: Leopold Lodewijk Filips Maria Victor; 9 April 1835 17 December 1909) was the second King of the Belgians from 1865 to 1909 and, through his own efforts, the owner and absolute ruler of the Congo Free State from 1885 to 1908.

Born in Brussels as the second but eldest surviving son of Leopold I and Louise of Orlans, he succeeded his father to the Belgian throne in 1865 and reigned for exactly 44 years until his death, the longest reign of any Belgian monarch. He died without surviving legitimate sons. The current Belgian king descends from his nephew and successor, Albert I.

Leopold was the founder and sole owner of the Congo Free State, a private project undertaken on his own behalf as a personal union with Belgium. He used Henry Morton Stanley to help him lay claim to the Congo, the present-day Democratic Republic of the Congo. At the Berlin Conference of 18841885, the colonial nations of Europe authorised his claim and committed the Congo Free State to him. Leopold ran the Congo by using the mercenary Force Publique for his personal gain. He extracted a fortune from the territory, initially by the collection of ivory and, after a rise in the price of natural rubber in the 1890s, by forced labour from the native population to harvest and process rubber.

Leopold's administration of the Congo Free State was characterized by atrocities and systematic brutality, including torture; murder; and the amputation of the hands of men, women and children when the quota of rubber was not met. In 1890, George Washington Williams used the term "crimes against humanity" to describe the practices of Leopold's administration of the Congo Free State.These and other facts were established at the time by eyewitness testimony, on-site inspection by an international commission of inquiry, and the 1904 Casement Report. Modern estimates range from 1 million to 15 million Congolese deaths, with a consensus growing around 10 million. Some historians argue against these figures by citing the lack of reliable censuses, the enormous mortality caused by smallpox and African trypanosomiasis and the fact that there were only 175 administrative agents in charge of rubber exploitation. In 1908, the reports of deaths and abuse and pressure from the Congo Reform Association and other international groups induced the Belgian government to take over the administration of the Congo from Leopold as a new territory, Belgian Congo.

The Congo Free State, also known as the Independent State of the Congo (French: État indépendant du Congo), was a large state and absolute monarchy in Central Africa from 1885 to 1908. It was privately owned by and in a personal union with Leopold II of Belgium; it was not a part of Belgium, of which he was the constitutional monarch. Leopold was able to seize the region by convincing other European states at the Berlin Conference on Africa that he was involved in humanitarian and philanthropic work and would not tax trade. Via the International Association of the Congo, he was able to lay claim to most of the Congo Basin. On 29 May 1885, after the closure of the Berlin Conference, the king announced that he planned to name his possessions "the Congo Free State", an appellation which was not yet used at the Berlin Conference and which officially replaced "International Association of the Congo" on 1 August 1885. The Congo Free State operated as a separate nation from Belgium, in a personal union with its King, privately controlled by Leopold II, although he never personally visited the state.The state included the entire area of the present Democratic Republic of the Congo and existed from 1885 to 1908, when the government of Belgium reluctantly annexed the state after international pressure.Leopold's reign in the Congo eventually earned infamy on account of the atrocities perpetrated on the locals. Leopold II's Free State extracted ivory, rubber and minerals in the upper Congo basin for sale on the world market through a series of international concessionary companies, even though its ostensible purpose in the region was to uplift the local people and develop the area. Under Leopold II's administration, the Congo Free State became one of the greatest international scandals of the early 20th century. The Casement Report of the British Consul Roger Casement led to the arrest and punishment of officials who had been responsible for killings during a rubber-collecting expedition in 1903.The loss of life and atrocities inspired literature such as Joseph Conrad's novel Heart of Darkness and raised an international outcry. Debate has been ongoing about the high death rate in this period. The boldest estimates state that the forced labour system led directly and indirectly to the deaths of 50 percent of the population. The lack of accurate records makes it difficult to quantify the number of deaths caused by the exploitation and the lack of immunity to new diseases introduced by contact with European colonists. During the Congo Free State propaganda war, European and US reformers exposed atrocities in the Congo Free State to the public through the Congo Reform Association, founded by Roger Casement and the journalist, author, and politician E. D. Morel. Also active in exposing the activities of the Congo Free State was the author Sir Arthur Conan Doyle, whose book The Crime of the Congo was widely read in the early 1900s. By 1908, public pressure and diplomatic manoeuvres led to the end of Leopold II's absolutist rule and to the Belgian Parliament enacting an act to annex the Congo Free State as a colony of Belgium. It became known thereafter as the Belgian Congo. In addition, a number of major Belgian investment companies pushed the Belgian government to take over the Congo and develop the mining sector as it was virtually untapped.